The filarial parasite Onchocerca volvulus is endemic throughout sub-Saharan Africa. Several lines of evidence suggest that different strains of the parasite exist in the forest and savannah bioclimes. In conjunction with the existence of at least two strains of O. volvulus, the parasite is transmitted by at least six sibling species of the Simulium damnosum complex. This has led to the hypothesis that specific vector-parasite complexes exist, a hypothesis that has significant implications for the strategic planning of efforts to control onchocerciasis in West Africa. As an outgrowth of the development of O. volvulus strain specific DNA probes, a 150 base pair (bp) repeated sequence family, designated Ovl5O, has been characterized, and has been found to be divergent in forest and savannah derived parasites. The objective of this proposal is to use the divergence of the Ovl5O family to test the hypothesis that specific vector-parasite complexes exist. O. volvulus infected blackflies will be collected, and the species of each fly determined. The Ovl5O family will be amplified from parasites infecting each species of fly, using the polymerase chain reaction. The DNA sequence of several representatives of the Ovl5O family from each isolate will be determined, and analyzed to determine if the parasite populations infecting the different vector species are distinguishable. If the populations are found to differ, the sequence data will be used to construct phylogenetic networks for the distinguishable populations. DNA sequence data from additional loci will also be collected to supplement the phylogeny produced by the Ovl5O analysis. The DNA sequence of a segment of the mitochondrial 16s rRNA of the vector species will also be determined, and used to construct phylogenies for the vector species, using numerical taxonomic and parsimony methods. Topologies of the vector and parasite phylogenies will be compared to test the hypothesis of co-speciation of the vector-parasite complexes.